1. Technical Field of the Invention
The present invention generally relates to the field of electronic measurement. In particular, the present invention provides a system and method for self-calibrated measurement for measuring values of such things as temperature.
2. Description of the Related Art
Many means of performing electronic measurement are known and are used in a variety of applications. Such means of performing temperature measurement are included in temperature control systems that accurately measure and control temperature during various chemical processes. For example, processing equipment, such as fluidics stations or processing chambers (e.g., furnaces), may be used in preparing, testing, and/or using semiconductors and polymer arrays. These systems typically regulate, and therefore measure, the temperature of an internal chamber. Further, these systems often include temperature sensitive devices (TSDs), such as Resistance Temperature Detectors (RTDs), integrated into the processing equipment to sense the temperature of zones in the processing equipment so that the product being processed is at the proper temperature. Employment of an RTD in an application provides ease of use and a nearly linear response curve over the temperature range of interest. However, circuits used to determine the temperature of the RTD often have a drawback in that they may require calibration that involves removal of the instrument from the processing equipment and setting potentiometers with the use of an external device for temperature measurement. Thus, these circuits require tedious manual calibration that may require the equipment to be disassembled.
To overcome this tedious manual calibration process, self-calibrating temperature measurement circuits have been proposed, for example, the circuits in U.S. Pat. No. 5,519,644, U.S. Pat. No. 4,210,024, U.S. Pat. No. 4,125,023, and U.S. Pat. No. 4,532,601. However, there is a need for improved self-calibrating temperature measurement devices and methods which have a simplified design and measurement process.
The present invention provides systems and methods for self-calibrated measurements, for example temperature sensing. The systems include a controller, a multiplexer of low on-resistance, at least two calibration reference resistors, and a current-to-frequency converter, and they perform self-calibrated temperature sensing with temperature sensing devices such as resistance temperature detectors (RTDs). In general, the system provides self-calibrated temperature sensing by the current-to-frequency converter providing a constant voltage sequentially to at least two calibration resistors and one or more RTDs using switches of low on-resistance in the multiplexer, which is controlled by the controller. The value of the one of the reference resistors provided is correlate to the resistive value of the RTD at the minimum temperature of the operating temperature range of, for example, the processing equipment to be monitored, and the value of the other reference resistor provided is correlate to the resistive value of the RTD at the maximum temperature of the operating temperature range of, for example, the processing equipment. The frequency output by the current-to-frequency converter varies directly with current variation, and thus, inversely with the resistance of the individual calibration resistors or RTDs. The controller determines composite resistances for the calibration resistors and RTDs inclusive of the other component(s) connecting the calibration resistors and RTDs to the current-to-frequency converter from the frequencies output by the current-to-frequency converter. A calculation is performed using these resistances to determine an accurate self-calibrated temperature for the RTDs and the equipment to.which they are associated.
In a first variation of the invention, the measurement system has at least two precision resistors, a current-to-frequency converter, and a multiplexer of low on-resistance. The measurement system produces an accurate temperature measurement of, for example a processing chamber, which is independent of multiplicative effects and additive effects of the temperature measurement circuitry by measuring frequencies when the current-to-frequency converter is connected to a first reference resistor, a second reference resistor, and an RTD. The controller then calculates the reciprocals of these frequencies to derive values proportional to the respective composite resistances, and from these values calculates a self-calibrated temperature of the RTD. Further, the temperature measurement circuit is calibrated on a continuous basis so that the remainder of the circuitry can be made up of components whose values are not precisely known and whose drift characteristics need not be tightly controlled.
In another variation of the present invention the current-to-frequency converter includes a voltage-to-frequency converter configured so that its output frequency varies in response to variations in current through a resistor connected to one of its terminals.
Another variation of the present invention provides.the measurement system may be constructed so as to provide self-calibrated temperature measurements of one or more locations in a processing chamber which may operate within different temperature ranges or to provide self-calibrated temperature measurements of, for example, one or more processing chambers. This capability may be provided by including multiple calibration reference resistors, a different set of two being associated with calibrating RTDs used with different temperature ranges.
A still further variation of the present invention enables even more accurate self-calibrated temperature measurements by utilizing multiple reference resistors so as to provide piecewise linear calibration within the temperature operating range of the processing equipment. As such, the values of the plurality of reference resistors provided are correlate to the resistive value of the RTDs at various points within the operating temperature range.